Lamp monitoring apparatus



May 18, 1965 Y s l 3,184,637

LAMP MONITORING APPARATUS Filed Dec. 10, 1962 United States Patent Oil ice 3,l$4,h37 Patented May 18, 1965 3,184,637 LAIWP MONlTURlNG APPARATUS Peter Skinner, London, England, assignor to Decca Lrm= ited, London, England, a British company Filed Dec. 1%, 1962, Ser. No. 243,230

Claims priority, application Great Britain, Dec. 13, 1961,

44,545/61 S (Ilaims. (ill. 315-429) This invention relates to apparatus for providing an electric signal indicating when a fluorescent lamp is on or when a fluorescent lamp is oil.

According to this invention, in lamp monitoring apparatus for monitoring a fluorescent lamp having a trans former winding connected in shunt across the fluorescent lamp and a ballast choke connected in series with the lamp in an alternating supply circuit, monitoring means are provided to give an output responsive to the alternating voltage across the ballast choke and/or the transformer. In a fluorescent lamp circuit having the lamp in shunt across a transformer winding and having a series ballast choke, when alternating power is applied to the circuit, if the lamp has not struck, there is only a small current drawn from the supply and there is therefore only a small drop in potential across the ballast choke and substantially full voltage across the transformer. The transformer is comrnonly an auto-transformer and, in this condition with full voltage across the transformer winding, tapped portions of the winding can provide the necessary heater voltage for the heaters for the lamp. As soon as the lamp strikes, it passes current and the voltage across the ballast choke rises and that across the transformer drops. As a typical example in a lamp operated from a 240 volt alternating supply, the voltage across the lamp might drop to as low as 50 volts as soon as the lamp has struck.

To monitor when the lamp is on most conveniently the monitor means are made responsive to the voltage across the ballast choke which voltage rises as soon as the lamp is on. If the lamp, for any reason should fail and not draw any current, the voltage across the ballast choke will drop. The monitoring means thus provides a positive indication that the lamp is drawing current.

The monitor means conveniently comprises a rectifier and detecting system for producing a direct voltage responsive to the alternating voltage across the ballast choke or transformer. Conveniently this voltage is derived by putting an auxiliary winding on the ballast choke or the transformer which auxiliary winding is connected to the aforementioned rectifier and detecting system.

The invention also includes within its scope a fluorescent lamp system having a fluorescent lamp with a transformer winding connected in shunt across the lamp and a ballast choke in series with the lamp in an alternating supply circuit wherein there is provided monitoring apparatus comprising an auxiliary winding inductively coupled to the lamp supply circuit and a rectifying circuit energised from said auxiliary winding to produce an output dependent on the voltage induced in said auxiliary winding.

The following is a description of two embodiments of the invention reference being made to the accompanying drawings in which FIGURES l and 2 are circuit diagrams illustrating respectively these two embodiments.

Referring to FIGURE 1 there is shown a fluorescent lamp it) which is energised from an alternating current supply at terminals 11, 12. In the known manner a transformer winding 13 is connected in shunt across the lamp and a ballast-choke 14 is connected in series with the lamp in the alternating supply circuit. The transformer winding 13 is tapped to provide the necessary al ternating supply for the heaters 15 of the lamp. If the fluorescent lamp is not struck, there is only a small current drawn from the supply and there is therefore only a small drop in potential across the ballast choke 14 and substantially the full supply voltage will appear across the transformer; the tapped portions of the winding thus provide in these circumstances the necessary heater voltage for the heaters. As soon as the lamp strikes, current will be drawn and so the voltage across the transformer will drop and the voltage across the ballast choke 14 will rise.

The present invention is concerned more particularly with monitoring such a lamp and, in the arrangement of FIGURE 1, .a monitoring circuit is provided for moniton ing the voltage across the transformer Winding 13 so as to provide an output signal if this voltage should rise which would happen if for any reason the lamp was no longer drawing current from the supply. The monitoring circuit includes an auxiliary winding 20 on the transformer l3 and the voltage developed across this winding is rectified by a rectifier 21 and capacitor 22 to provide a control signal which is applied to the base of a transistor 23. The transistor 23 has a load resistor 24 connected between the collector and a 12 volt supply terminal 25. The emitter of the transistor is grounded. The output signal is provided by the potential at the collector end of resistor 24 which point is connected to an output terminal 26. A resistor 27 between the base of the transistor 23 and the l2 volt supply terminal 25 biases the transistor so that it is substantially fully conductive in the absence of any applied signal to the base. In this particular embodiment of the invention, the turns ratio of the windings 13, 29 is such that if the lamp It) is not struck then the voltage across the winding 25) is greater than 13 volts peak to peak magnitude whereas, if the lamp has struck, the,

voltage is less than 7 volts peak to peak. The transistor 23 is operated with a 12 volt supply and the circuit is arranged so that the transistor, in the absence of any applied signal from the rectifier 21 is fully conductive due to base current drawn through the resistor 27. When the lamp has struck the voltage on the transformer wind ing is so small that the bias applied by a Zener diode 28 prevents the rectifier 21 conducting. The transistor 23 is thus substantially fully conductive and the terminal 26 will be at substantially ground potential. When the lamp is not struck, the voltage across the transformer winding 24 rises and overcomes bias applied to the rectifier 2E. The consequent signal applied to the base of the transistor 23 from the rectifier 21 causes the current drawn by the transistor 23 to drop substantially so that the voltage at the output terminal 26 will drop in a typical case to about 6 volts. It will be seen that the circuit of arrangement of FZGURE 1 provides a direct voltage output at terminal 26 with respect to earth which is substantially zero when the lamp is lit but gives a positive indication of lamp failure.

FIGURE 2 illustrates a modification of the circuit arrangement in FIGURE 1. The lamp It as before is connected in shunt across a transformer 13 which is tapped to feed the heaters 15 and is connected in series with a ballast choke 14. In the arrangement of FIGURE 2 an auxiliary winding 30 is coupled to the ballast choke 14. The voltage across this winding will be small when there is no current being drawn by the lamp and in a typical case the turns ratio would be made such that the peak to peak voltage across the winding 36 is less than 18 volts when the lamp is not lit. When the lamp is struck however there is current drawn through the choke 14 and the voltage across the winding 3i} would rise typically to a value greater than 45 volts peak to peak. This alternating signal is rectified by a rectifier 31 in series with a resistor 32. The direct voltage datum level of this signal is fixed by connecting the junction of the winding 3!) coil resistor 32 to a '+l-2 volts supply terminal 33. The voltage developed at the other end of the resistor 32 is applied through a rectifier 34 to the base of a transistor 35. This transistor has its emitter connected to a terminal 36 at ground potential and its collector connected via a load resistor 37 to'a terminal 38 at 12 volts. The output signal is developed across the load resistor 37 and is taken from. a terminal 39 connected to the collector. A smoothing" capacitor 46 is connected between ground and the junction of resistor 32 and rectifier 31. When the lamp is struck and a relatively large negative voltage is developed across the capacitor 49, the diode 34 is nonc-onductive and the base current applied via a resistor 41 between the base of the transistor 35 and the 12 volt supply terminal 38 makes the transistor become fully conductive. The potential at the terminal 39 therefore rises substantially to ground potential. If the lamp 10 should fail for any reason so that little or no current is drawn from the supply, the potential across the winding will fall and hence the diode 34 conducts and the resistor 32 applies reverse bias to the transistor which cuts off and, in this particular embodiment, the potential at the collector terminal 39 will fall to about 8 volts. The circuit of FIGURE 2 again provides an output potential with respect to ground giving a positive indication when the lamp has failed. The arrangement of FIGURE 2 however has the further advantage over the circuit of FIGURE 1 in that the output requires a decrease in voltage across the reservoir capacitor for fault indication and hence is much less sensitive to interference than the arrangement of FIGURE 1 where the reservoir capacitor 22 is connected between the base and the emitter of the transistor so that very small noise signals will affect the charge on this capacitor.

I claim:

1. In a fluorescent lamp system having a fluorescent lamp with a transformer winding connected in shunt across the lamp and a ballast choke in series with the lamp in an alternating supply circuit, monitoring apparatus comprising an auxiliary winding inductively coupled to the lamp circuit to have an induced alternating signal of magnitude dependent on the relative potential distribution across the ballast choke and transformer winding, a rectifier arranged to produce a direct control signal of magnitude dependent on the magnitude of said induced alternating signal, an auxiliary supply source, an output circuit, and switch means controlled by said control signal and controlling the application of potential from said auxiliary supply source to said output circuit, said switch means being arranged to reduce the potential in the output circuit unless the control voltage corresponds to a potential distribution in the lamp circuit with substantially the full alternating supply voltage across the transformer.

2. Monitoring apparatus as claimed in claim 1 wherein the auxiliary winding is inductively coupled to the ballast choke. I

3. In a fluorescent lamp system having a fluorescent lamp with a transformer winding connected in shunt across the lamp and a ballast choke in series with the lamp in an alternating supply circuit, monitoring apparatus comprising an auxiliary winding inductively coupled to the lamp circuit to have an induced alternating signal of magnitude dependent on the relative potential distribution across the ballast choke and transformer winding, biasing means responsive to said induced alternating signal and a rectifier connected to said biasing means which rectifier is arranged to produce a direct control signal of magnitude dependent on the bias applied by said biasing means and on the magnitude of said induced alternating signal, an auxiliary supply source, an output circuit, and switch means controlled by said control signal and controlling the application of potential from said auxiliary supply source to said output circuit, said switch means being arranged to reduce the potential on the output circuit unless the control voltage corresponds to a potential distribution in the lamp circuit with substantially the full alternating supply voltage across the transformer.

4. In a fluorescent lamp system having a fluorescent lamp with a transformer winding connected in shunt across the lamp and a ballast choke in series with the lamp in an alternating supply circuit, monitoring apparatus comprising an auxiliary winding inductively coupled to the lamp circuit to have an induced alternating signal of magnitude dependent on the relative potential distribution across the ballast choke and transformer winding, biasing means responsive to said induced alternating signal and a rectifier connected to said biasing means which rectifier is arranged to produce a direct control signal of magnitude dependent on the bias applied by said biasing means and on the magnitude of said induced alternating signal, an auxiliary supply source, a transistor Whose conductivity is arranged to be reduced by said control signal, said transistor being provided with transistor biasing means causing the transistor to conduct in the absence of a control signal and an output circuit connected through the transistor to one potential in the auxiliary supply source and through a resistance to a second potential in the auxiliary supply source.

5. Monitoring apparatus as claimed in claim 4 wherein the auxiliary winding is inductively coupled to the ballast choke.

References Cited by the Examiner UNITED STATES PATENTS DAVID J. GALVIN, Primary Examiner. 

1. IN A FLUORESCENT LAMP SYSTEM HAVING A FLURESCENT LAMP WITH A TRANSFORMER WINDING CONNECTED IN SHUNT ACROSS THE LAMP AND A BALLAST CHOKE IN SERIES WITH THE LAMP IN AN ALTERNATING SUPPLY CIRCUIT, MONITORING APPARATUS COMPRISING AN AUXILIARY WINDING INDUCTIVELY COUPLED TO THE LAMP CIRCUIT TO HAVE AN INDUCED ALTERNATING SIGNAL OF MAGNITUDE DEPENDENT ON THE RELATIVE POTENTIAL DISTRIBUTION ACROSS THE BALLAST CHOKE AND TRANSFORMER WINDING, A RECTIFIER ARRANGED TO PRODUCE A DIRECT CONTROL SIGNAL OF MAGNITUDE DEPENDENT ON THE MAGNITUDE OF SAID INDUCED ALTERNATING SIGNAL, AN AUXILIARY SUPPLY SOURCE, AN OUTPUT CIRCUIT, AND SWITCH MEANS CONTROLLED BY SAID CONTROL SIGNAL AND CONTROLLING THE APPLICATION OF POTENTIAL FROM SAID AUXILIARY SUPPLY SOURCE TO SAID OUTPUT CIRCUIT, SAID SWITCH MEANS BEING ARRANGED TO REDUCE THE POTENTIAL IN THE OUTPUT CIRCUIT UNLESS THE CONTROL VOLTAGE CORRESPONDS TO A POTENTIAL DISTRIBUTION IN THE LAMP CIRCUIT WITH SUBSTANTIALLY THE FULL ALTERNATING SUPPLY VOLTAGE ACROSS THE TRANSFORMER. 